As a method and instrument for measuring diabetic patients' blood sugar level, namely, glucose concentrations in blood, a method and instrument using absorptions in near-infrared regions have been known. However, they have not been used widely due to the level of their accuracy and difficulties in their measuring methods. On the other hand, a method and instrument for measuring glucose concentrations in blood using infrared spectroscopy have been proposed, which are described in Japanese Patent Laid-Open Publication No. 2003-042948 (hereafter referred to as patent document 1)